The present invention relates to a magnetostrictive sensor for measuring torque in a shaft, wherein the sensor comprises at least one active magnetostrictive region of the shaft, a surface pattern on the magnetostrictive region such that it obtains anisotropic properties, a first means arranged to generate a magnetic field varying in time in the magnetostrictive region and a second means arranged to sense variations in the permeability in the magnetostrictive region, and wherein said magnetostrictive region comprises a first layer of a magnetostrictive material provided on the surface of the shaft.
Such magnetostrictive sensors are known and exist in a number of different embodiments. Usually, magnetostrictive sensors according to the above comprise first means comprising a first winding provided in a yoke and extending around the magnetostictive region of the shaft. A current varying in time is arranged to be supplied to the first winding such that a magnetic field varying in time is generated in the magnetostrictive region. The second means comprises in general a second winding, which is provided in the same yoke as the first winding. Hereby, a voltage is induced in the second winding with a value in proportion to the magnetic flux density. Since the permeability of the magnetostrictive region is changed when it is subjected to a torque, the magnetic flux density is also influenced. The voltage induced in the second winding may thereby be used for determining the magnitude of the torque in the shaft.
The differences between different known magnetostrictive sensors are principally the design of the magnetostrictive region and the way to obtain anisotropic properties therein.
U.S. Pat. No. 5,646,356 shows a magnetostrictive sensor for measuring of torque in a shaft. Thin strips of a low resistivity material have been applied to the surface of the shaft having an angle of 45xc2x0 to the extension of the shaft. The use of this sensor is restricted to measure torque in shafts which consist of materials having good magnetostrictive properties. Since drive shafts only in exceptional cases are manufactured of material having good magnetostrictive properties, the use of this sensor is restricted.
U.S. Pat. No. 5,491,369 shows a magnetostrictive sensor for measuring a torque applied to a shaft. In order to provide such a sensor on a shaft, a plurality of grooves are formed on the circumferential surface of the shaft. Then, the shaft is subjected to a heat treatment such that it receives an increased strength. Thereafter a binder layer is applied before an active magnetostrictive material is provided on the circumferential surface of the shaft. Consequently, the method requires both mechanical treatment and heat treatment of the load-carrying shaft, which makes it less attractive for many applications.
JP 4-221 726 shows a magnetostrictive sensor for measuring torque in a shaft. The sensor comprises a magnetostrictive region having a first layer of nickel which abuts the surface of the shaft and a second layer of permalloy, which is a ferro-magnetic material having a very high permeability, provided on the first layer. After the application of the second layer on the first layer, fine strips of the second layer are removed such that the magnetostrictive region obtains a surface pattern, which gives the magnetostrictive region anisotropic properties.
The second layer must be magnetostrictive for the function of the sensor and a considerable disadvantage is that the second layer also must have a high permeability.
CN 1030642 shows a magnetostrictive torque sensor having a first layer of copper provided on top of a circumferential surface of a shaft and thin strips of a magnetostrictive alloy provided thereon. Also in this case, the applied strips must have a high permeability for the function of the sensor.
JP 10-176 966 shows a magnetostrictive torque sensor which has a first layer of a magnetostrictive material which is provided on a circumferential surface of a shaft. The magnetostrictive sensor here described is based on geometric anisotropy, which is provided since the first layer forms a geometric pattern on the surface of the shaft. The object of this invention is to improve the strength of the first layer regarding breakage and separation. Therefore, the first layer is provided with a gradually decreasing thickness towards its end portions. Thereafter, a second layer of a non-magnetostrictive material is provided such that it extends over the end portions of the first layer having a decreasing thickness. Thereby, the strength of the first layer is improved regarding breakage and separation. Consequently, the function of the second layer is only to provide a favourable distribution of mechanical stress in the first layer in a mechanical manner. In the same manner as the sensors according to the above-cited JP4-221 726 and CN 1030642, the applied strips require a high permeability for the function of the sensor.
The object with the present invention is to provide a magnetostrictive sensor for measuring a torque in a shaft, which sensor is simple to provide on a shaft, provides good measurement results and is possible to provide on existing shafts substantially independent of the manufacturing material of the shaft.
This object is achieved by the magnetostrictive sensor of the initially mentioned kind which is characterised in that said surface pattern is formed by a second layer of a non-magnetostrictive material comprising a low resistivity. Because of eddy currents induced in the first layer, the applied magnetic field decreases (is damped) exponentially with the distance from the surface of the first layer. By choosing a sufficiently thick layer, the properties of the sensor will be dominated by the first layer material and not by the shaft material. By providing a first layer of a suitable thickness on the circumferential surface of the shaft, the influence of the shaft material on the measurement results of the sensor becomes more or less negligible. Thereby, the sensor may be provided on substantially all kinds of shafts and independent of shaft material. Such a magnetostrictive sensor also obtains a good function with a first layer of a material having a relatively low permeability.
According to a preferred embodiment of the present invention, said second layer is provided on the first layer. By providing a second layer of a non-magnetostrictive material on the first layer, the surface pattern which provides the magnetostrictive region with anisotropic properties is obtained. Advantageously, said first layer has a continuous extension in said region. Such a first layer is simple to provide at the same time as it forms a continuous and even underlying surface on which the second layer may easily be provided. Furthermore, such a continuous first layer minimises stress concentrations in the magnetostrictive region. According to an alternative embodiment, the first layer is provided with a non-continuous extension on the surface of the shaft and in that at least any portion where the first layer does not abut the surface of the shaft, the second layer is provided on the surface of the shaft. By providing both the first layer and the second layer on the surface of the shaft in said surface pattern, the magnetostrictive region may obtain a substantially even surface layer.
According to another preferred embodiment said first and second layers are arranged to be applied by an application method which essentially does not introduce heat. By avoiding high temperatures, the strength and tolerances of the load-carrying shaft are not influenced. A suitable application method which does not introduce heat is plating. There exists at least two applicable plating methods for plating metals on an existing element, namely bath plating and selective plating, but also chemical (electro-less) plating is applicable. Advantageously, the surface of the shaft is arranged to be subjected to a pre-treatment before said layers are applied. By such a pre-treatment, which may comprise blasting, shot peening, grinding, pickling, doping or chemical strike, the first layer obtains a sufficient bond to the surface of the shaft. Advantageously, said applied layers are arranged to be subjected to an additional post-plating treatment. Such a treatment may be mechanical and/or thermal for improving the strength of the layers and the properties of the sensor.
According to another preferred embodiment of the present invention, said first layer has a thickness which is larger than the skin depth on the magnetostrictive material. The skin depth of a material is a well known definition in the present technical field and is calculated according to the present formula   δ  =                    2        ⁢                  xe2x80x83                ⁢        ρ                    μ        ⁢                  xe2x80x83                ⁢        ω            
where
xcex4=the skin depth,
xcfx81=the electric resistivity of the material,
xcexc=the magnetic permeability of the material and
xcfx89=the angular frequency of the applied field.
In a first layer having a thickness of a skin depth, about ⅓ of the applied magnetic field penetrates through the first layer into the underlying shaft material. In order to guarantee a good accuracy of measurement, it is preferable to apply such a thick layer that only a lesser amount of the applied magnetic field penetrates through the first layer into the shaft material. However, providing a first layer thickness which is greater than two skin depths usually contributes very little with respect to the accuracy of measurement since, at two skin depths, only a very small amount of the applied magnetic field penetrates into the underlying shaft material. A greater layer thickness implies in general a higher manufacturing cost. Therefore, a first layer having a thickness between one and two skin depths is optimal for most sensors. However, during circumstances favourable in other respects, a relatively good accuracy of measurement may be obtained also with a thickness of the first layer as little as xc2xc of the skin depth of the magnetostrictive material. Advantageously, said first layer comprises at least one or more of the materials nickel, iron or cobalt. In particular nickel has in a pure state or in alloys good magnetostrictive properties, in combination with that it may be applied without difficulty on most kinds of shaft materials by, for example, plating.
According to another preferred embodiment of the present invention, said second layer comprises a material having a resistivity which is lower than the resistivity of the material of the first layer. With reference to the above mentioned U.S. Pat. No. 5,646,356, it is shown that the quotient between the resistivity of the material in the second layer and the resistivity of the material in the first layer as well as the thickness of the second layer are of significance for how well the applied field is aligned with respect to the surface pattern of the second layer. It is suitable that said second layer has a thickness which is smaller than the skin depth in the first layer and larger than the skin depth multiplied by the quotient between the resistivity of the material in the second layer and the resistivity of the material in the first layer. Advantageously, a material in the second layer is chosen which has as low resistivity as possible. With a low resistivity, the second layer may be made very thin. Said second layer may comprise strips arranged in parallel which form an angle of between xc2x120xc2x0 and xc2x175xc2x0 to a generatrix to the surface of the shaft. For the magnetostrictive region to obtain an optimal anisotropy, said angle should be xc2x145xc2x0. At these angles, the magnetic field coincides at the shaft surface with the principal mechanical stress directions when the shaft is loaded with a torque in either direction. Said second layer may be provided within at least two zones, which comprise strips having different angles. The strips may, for example, have an angle of +45xc2x0 to said generatrix in a first zone while the strips have an angle of xe2x88x9245xc2x0 to said generatrix in a second zone. Said strips may extend axially in a continuous way between said zones. The total length of the sensor then becomes minimal. It is also possible that the strips are interrupted between two zones. Advantageously, said second layer comprises one or more of materials copper, aluminium or chrome. In particular copper has a very low resistivity but aluminium and chrome also have a low resistivity. Copper and chrome may be applied without difficulty on top of a first layer of, for example, nickel, by plating.